We have undertaken a project to generate models of human congenital defects by screening ENU- mutagenized mice for recessive mutations affecting late embryonic development. The screen incorporated a genetic mapping component to facilitate the positional cloning and functional characterization of the mutant genes. The strategy has worked well, and we have generated many mice with phenotypes similar to human malformation syndromes and birth defects. We have mapped most of these, and identified the mutated locus in over 40 lines. In addition to characterizing the biology of the defects in the mutant mice, we have in several cases established that the genes we identified play a role in the causation of human disease. Thus goals of our project to date have been met. In this renewal proposal we aim to continue this effort, taking advantage of substantial progress in relevant genomic and transgenic technology that will make this project even more productive. Most importantly, we plan to take advantage of methods we have developed for positional cloning using whole genome sequencing and ENU-induced variant mapping analysis, which we have shown can be successful even when mice are maintained on inbred genetic backgrounds. This will enable the ascertainment of modifier loci by allowing us to incorporate mutant lines into our screens, evaluate their progeny for enhancement or suppression of the baseline phenotypic abnormalities, and localize the interacting loci without the confounding effects of strain-specific variation. We are applying this strategy for investigation of bone formation and skeletal patterning, because of the considerable clinical significance of these disorders as well as the practicality of their analysis using our planned method.